Abstract

We report on the photoresponse of an asymmetrically doped p−-Ge/n+-Si heterojunction photodiode fabricated by wafer bonding. Responsivities in excess of 1 A/W at 1.55 μm are measured with a 5.4 μm thick Ge layer under surface-normal illumination. Capacitance−voltage measurements show that the interfacial band structure is dependent on both temperature and light level, moving from depletion of holes at −50 °C to accumulation at 20 °C. Interface traps filled by photo-generated and thermally-generated carriers are shown to play a crucial role. Their filling alters the potential barrier height at the interface leading to increased flow of dark current and the above unity responsivity.

Figures (4)

(a) Schematic illustration of the Ge/Si photodetectors made by wafer bonding followed by chemical-mechanical polishing. (b) HR-TEM image of the Ge/Si interface. The two zoomed-in images show the thin (~2 nm thick) interfacial layer (on the left) and the thick region (on the right).

(a) Dark current density (J) versus reverse bias voltage (left axis) and C−V characteristic at 100 kHz (right axis) of the Ge/Si diode. The inset shows the J−V characteristics at two different temperatures. (b) 1/C2 versus reverse bias voltage at 20 °C and −50 °C. The value of the built-in potential (Ψbi) is shown. The inset of part (b) shows the depletion width (WD) as a function of reverse bias voltage at 20 °C and −50 °C. The shaded region illustrates the effect of charges captured by the interface traps at 20 °C.

Schematic representation of the Ge/Si band diagram at equilibrium at (a) −50 °C, and (b) 20 °C. Ψbi and ΨBp are the built-in potential and the Fermi potential with respect to the midgap in the bulk of p-Ge, respectively. ΦB is the potential barrier height. In part (a), the Ge surface at the interface is in the “weak inversion” mode while in part (b) it is in the “accumulation” mode due to trap filling. The dashed lines in (a) and (b) are the intrinsic Fermi level. The inset of part (b) schematically illustrates the potential barrier lowering due to filling of acceptor traps by either temperature or light.

(a) Responsivity of the Ge/Si photodiode versus input optical power at a wavelength of 1.55 μm and V = −2 V at two temperatures. (b) Responsivity as a function of wavelength at a constant optical power of 40 μW at different reverse bias voltages and temperatures.